The present invention relates to a multilayer printed wiring board with an interstitial via hole structure.
A multilayer printed wiring board having an interstitial via hole structure is conventionally fabricated in the following manner. Via holes are formed in a one-side copper-clad laminate so as to extend through an insulating substrate. Each via hole is filled with an electrically conductive paste or a metal conductor by means of copper plating so that the paste or conductor projects from a surface of the insulating substrate. On the other hand, a copper foil is formed by etching into pads on another printed wiring board to be laminated. Both printed wiring boards are bonded together by thermocompression bonding with an adhesive agent such as epoxy being interposed therebetween. As a result, both printed wiring boards are laminated with the adhesive agent therebetween, and a distal end of each metal conductor of the one printed wiring board is brought into contact with the corresponding pad of the other printed wiring board so that the printed wiring boards are electrically connected together.
In the foregoing fabricating method, however, the printed wiring boards are electrically connected together by a mechanical contact between each metal conductor slightly projecting from the printed wiring board and the pad. Accordingly, reliability should be still improved. A method is provided for electroplating the distal end of each metal conductor with a metal with a low melting point, such as Sn/Pb eutectic solder, for improvement in the reliability. However, since a solder melting at a relatively low temperature is used for connection, the reliability is reduced when the printed wiring board undergoes thermal history such as exposure to high temperature. Moreover, since the solder contains lead, the foregoing method is not preferable from the aspect of an environmental issue.
A first invention to solve the foregoing problem is a printed wiring board in which a via hole formed in an insulating substrate is filled with a metal conductor, the printed wiring board being laminated with another printed wiring board, and heated and pressed so that a distal end of the metal conductor is abutted against a conductor circuit formed on a surface of said another printed wiring board so that the printed wiring board is electrically connected to said another printed wiring board, characterized in that at least the distal end of the metal conductor or the surface of the conductor circuit has a metal having a melting point higher than a temperature at which both printed wiring boards are pressed and diffusing into the counter metal during press of the printed wiring boards.
A second invention is characterized in that the conductor circuit formed on the surface of said another printed wiring board or the metal conductor comprise copper and the distal end of the metal conductor is covered with a diffusing metal layer comprising tin.
A third invention is characterized in that the via hole is filled with the metal conductor by an electroplating.
A fourth invention is characterized in that the printed wiring boards described in any one of claims 1 to 3 are bonded together by the thermocompression bonding so that the distal end metal of one printed wiring board is diffused into the conductive circuit metal of the other printed wiring board such that both printed wiring boards are electrically connected to each other.
A fifth invention is a method of fabricating a multilayer printed wiring board, in which a plurality of printed wiring boards are bonded together by thermocompression bonding in a laminated state to be integrated, and a distal end of the metal conductor is pressed against a conductor circuit formed on a surface of said another printed wiring board so that the printed wiring board is electrically connected to said another printed wiring board, characterized by the step of forming a via hole in an insulating substrate of said one printed wiring board, the step of filling the via hole with a metal conductor, the step of forming a diffusing metal layer comprising a metal having a melting point higher than a temperature at which both printed wiring boards are bonded together by the thermocompression bonding and diffusing into the counter metal during the thermocompression bonding of the printed wiring boards, and the step of bonding both printed wiring boards by thermocompression bonding with the metal conductor being pressed against the conductor circuit of the counter printed wiring board.
A sixth invention is characterized in that a surface roughening process is applied to either one or both of the distal end of the metal conductor and the surface of the conductor circuit before the thermocompression bonding of both printed wiring boards.
According to the first invention of the printed wiring board and the fifth invention of the method of fabricating the multilayer printed wiring board, the printed wiring board is laminated with another printed wiring board, and both printed wiring boards are bonded together by the thermocompression bonding while the metal conductor of one board is pressed against the conductor circuit of the other board. In this case, pressure acting upon an interface causes diffusion of the metal even if at least the distal end of the metal conductor or the surface of the conductor circuit comprises a metal having a melting point higher than the temperature of thermocompression bonding. As a result, metal junction having a high strength and good electrical characteristic can be obtained. Accordingly, even when the multilayer printed wiring board undergoes thermal history such as exposure to high temperature, the junction is hard to be loosened and high reliability is obtained in the electrical connection. In this case, the metal diffusion takes place earlier when either one or both of the surfaces of the metal conductor and conductor circuit are roughened. Further, when the surface roughening process is previously applied to the contact surface, a contacting surface area is increased such that the metal diffusion is rendered smoother.
In the second invention, the conductor circuit and the metal conductor are made from copper and accordingly each has a low resistivity. Moreover, tin on the surface of copper of one element diffuses into copper such that a copper-tin alloy layer having a high melting point is formed. Further, according to the printed wiring board of claim 3 in which the via hole is filled with the metal conductor by the electroplating, the density of the metal conductor is increase such that the resistivity is reduced.
In each invention, the insulating substrate is preferably a glass cloth epoxy resin substrate, glass cloth bismaleimidetriazine resin substrate, glass cloth polyphenylene ether resin substrate or aramid unwoven fabric-polyimide resin substrate. The adhesive agent bonding the printed wiring boards in the laminated state is preferably a thermosetting epoxy adhesive. In this case, the temperature preferably ranges between 180xc2x0 C. and 230xc2x0 C., and the pressure preferably ranges between 10 and 60 kg/cm2. The thermocompression bonding is preferably carried out under reduced pressure.